1. Field of the Disclosure
The present application relates to an organic light emitting display device, and more particularly to a host material and an organic light emitting display device that are adapted to enhance efficiency and lifespan.
2. Description of the Related Art
Nowadays, as the information society grows, display devices have been rapidly developed. Among the display devices, OLED devices are a self-illuminating display device not requiring a backlight unit. As such, the OLED devices can become thinner. Also, the OLED devices can have low power consumption.
The organic light emitting display device includes an anode electrode, a cathode electrode and a light emission layer interposed between the anode and cathode electrodes. The anode electrode applies holes to the light emission layer, and the cathode electrode applies electrons to the light emission layer. In accordance therewith, excitons are generated in the light emission layer through the recombination of electrons and holes. The exciton emits light in transition to the ground state.
The excitons can be classified into singlet and triplet excitons. The singlet and triplet excitons exist in a ratio of 1:3. In order to emit light, fluorescence uses only the singlet excitons but phosphorescence uses both the singlet and triplet excitons. As such, the organic light emitting display device using a phosphorescent material can have a higher luminous efficiency compared to that using a fluorescent material. In other words, the phosphorescent material having higher quantum efficiency than that of the fluorescent material can increase the luminous efficiency of the organic light emitting display device. In view of this point, a variety of phosphorescent materials to be used in the organic light emitting display device are being researched.
The phosphorescent materials each include a host material and a phosphorescent dopant material capable of emitting light using the transition energy from the host material. In order to generate an energy transition phenomenon without causing a backward energy transition, a triple energy level of the phosphorescent dopant must be sufficiently greater than that of the host material. On the contrary, when the triplet energy level of the host material is higher than that of the phosphorescent dopant, the quantum efficiency of the host material cannot be maximized.
However, the triplet energy level of the host material according to the related art is lower than that of the phosphorescent dopant. Due to this, the quantum efficiency of the host material cannot be enhanced without limit.
Also, the related art host material has a low glass transition temperature. As such, the host material must be affected in thermal stability. Furthermore, the host material can be easily deformed at a high temperature or a driving temperature. As a result, the life span of a device including the host material can be reduced.
In view of these points, it is necessary to develop new host materials which are adapted to enhance the luminous efficiency and the life span of the organic light emitting display device.